Various certification methods for judging the authenticity of correspondents have been in existence for a long time. Many certification methods suitable to a communication system utilizing an open network for the general public such as the Internet have been developed in recent years. A digital sign method using public key cryptography is one kind of certification method, which is widely used. In the digital sign method, a sender who enciphers a plaintext by a secret key owned exclusively by the sender, transmits the enciphered text, which a recipient decrypts using the sender's public key. If the decryption is successful, the recipient can be certain that the decrypted plaintext was transmitted by the sender.
A successful decryption through a correct matching of the secret key and the public key can be achieved; however, in order to ensure that a high level of security is maintained, it is necessary to guarantee that the public key belongs to the real sender. This guarantee is realized by a public key certification, which is enciphered by the secret key owned exclusively by an impartial party, i.e., a Certifying Authority. That is to say, in the instance the recipient holds a public key of the Certifying Authority, and the sender transmits the above enciphered data along with the sender's own public key certificate acquired from the Certifying Authority, the recipient firstly verifies the authenticity of the public key certificate using the Certifying Authority's public key. And secondly, decrypts the enciphered data using the sender's public key included in the authenticated public key certificate. Sender's public key used here is guaranteed by Certificate Authority. Therefore the success of the decryption on the above-enciphered data means nothing else than a guarantee by Certificate Authority that the sender of the enciphered data is authentic.
The public key certificate issued by the Certifying Authority includes the date and time information on the validity period of the certificate; and the receiver of the above enciphered data and the public key certificate used by the recipient determines whether the present time is within the validity period in the public key certificate with reference to a clocking present time in the receiver. The receiver naturally determines that the public key certificate is authentic, if the present time is within the validity period of the public key certificate.
Precise clocking of the receiver is necessary to maintain a high level of security since imprecise clocking makes an inprecise judgement regarding the validity period of the public key. Deviations in the clocking present time of communication devices, such as currently existing personal computers, inevitably occur and deviations are gradually enlarged, even though a user of the communication device may start clocking at the precise present time. In other cases, a user may forget the initialization of the clocking time, or a completely false in the initialization. In such a case, wrong present time is clocking in the receiver. Without a precise clocking of the present time it is impossible to judge with accuracy, the validity period of a public key certificate. The problem of the incorrect clocking of present time affects not only the authenticity of the public key cryptography method, but uniformly affects all certifying methods having a validity period for the certificate.